The present invention relates to amplifier circuits for connecting the output terminals of a voltage source across a load impedance.
The operation of many electrical and electronic systems relies on the application of a voltage across a load from a voltage source and it is recognized in the art that the quality of performance of such systems depends on the extent to which the voltage across the load corresponds to, or equals, the open circuit source voltage, or an amplified version of the source voltage.
Ideal coupling would result in appearance of a voltage proportional to the open circuit source voltage across the load. This would cause the signal across the load to have exactly the same time and frequency domain characteristics as the source signal.
However, there is no circuit arrangement which achieves perfect transfer, or ideal coupling, of the source voltage to the load.
Errors in such voltage transfers have two basic causes. Firstly, a practical voltage source behaves like an ideal voltage source in series with a source impedance. Therefore, when current is drawn from such a source, there is a voltage drop across the source resistance. Secondly, the voltage source is connected to the load via a coupling medium which has a finite impedance at any given frequency, and which will therefore be responsible for a further voltage drop.
The achievement of ideal coupling would be advantageous in many electronics systems. For example, if the signal source is an electronic or electrical circuit port, the load is an oscilloscope employed to monitor the time domain waveform of the voltage developed by the signal source and the intervening coupling medium is the oscilloscope probe and associated cabling, the oscilloscope trace will accurately reflect the waveform of the source voltage only if the voltage applied to the oscilloscope inputs is identical to the open circuit source voltage.
For many other types of electronics purposes, ideal coupling would produce optimum results. This would also be the case in a variety of audio, data transmission, communication and telecommunication systems.
It is known to minimize source-to-load voltage drops, or signal attenuation, by the provision of an active circuit, known as an amplifier stage, between the source and load circuits. When such a circuit is used, for example as a unity gain voltage follower, the ratio of load voltage, V.sub.L, to source voltage, V.sub.i, can be expressed as follows: ##EQU1## Where A.sub.V is the open circuit voltage gain of the amplifier stage,
R.sub.IN is the driving point input resistance of the amplifier stage, PA1 R.sub.OUT is the driving point output resistance of the amplifier stage, PA1 Z.sub.i is the source impedance, and PA1 Z.sub.L is the load impedance.
If R.sub.IN is much greater than the absolute value of Z.sub.i, the absolute value of Z.sub.L is much greater than R.sub.OUT, and A.sub.V is approximately equal to 1, the load-to-source voltage ratio will be almost equal to unity. An ideal amplifier will have a value for R.sub.IN approaching infinity, a value for R.sub.OUT approaching zero and a value for A.sub.V substantially equal to unity.
However, practical amplifiers, when connected as a unity gain voltage follower or buffer, which include bipolar emitter followers and MOSFET source followers, have characteristics which are far from ideal. Emitter followers typically establish driving point input resistances that are rarely larger than a few hundred k.OMEGA. and driving point output resistances that are rarely smaller than several tens of .OMEGA.. Additionally, their open circuit voltage gains are usually no better than 0.95. MOSFET source followers provide a reasonable approximation of an infinitely large driving point input resistance, but their output resistance can be of the order of 100.OMEGA.. Moreover, the low frequency open circuit voltage gain of a MOSFET source follower can be as low as 0.75 and when compared with bipolar emitter followers, the frequency response of a MOSFET source follower is substantially inferior. Both of these types of followers have marginal high frequency response capabilities. In the case of an emitter follower circuit, the frequency response can also be significantly underdamped, which would promote circuit and system instability, particularly when the load is highly capacitive.
It is also known in the art to employ an operational amplifier connected, for example as a voltage follower, to transfer a voltage from one circuit point to another. Known circuits of this type do not approximate ideal coupling.